The present invention relates to rotary printing machines and more particularly to plate clamping devices for clamping flexible printing plates on printing rollers of rotary printing machines.
The clamping devices for clamping printing plates are normally known in the art. These devices include a rigidly mounted lower guiding rail and an upper guiding rail which are merely connected to each other by means of bolts having rounded heads in such a fashion that the upper rail may pivot relatively to the lower rail. These devices include an eccentric shaft extended over the entire length of the upper rail and positioned between the upper and lower rails and operated by a plug spanner. The clamping action between the upper rail and the lower rail after a printed plate has been inserted therebetween is attained by rotation of the eccentrical shaft which generates the pivoting movement of the upper rail between its open position and closed positions. The opening movement of the clamping device is effected by means of compression springs mounted between the upper and lower guiding rails. This is disclosed in the U.S. Pat. No. 3,156,184.
The disadvantage of this otherwise satisfactory device is that because of different distribution of the clamping force over the entire length of the clamping device the unavoidable deviation from parallelism between the premanufactured upper and lower guiding rails occurs in prior art device. Even by adjustment of the lower and upper guiding rails parallel to one another the parallelism between the eccentrical shaft and the clamping force application surface of the upper rail cannot be obtained.
Because the clamping force usually results in elastic deformation even substantially small mass deflections become effective.
A further disadvantage of the prior art devices resides in that the clamping force is always increased in the vicinity with bolts mounted between the upper and lower guiding rails. This is particularly disadvantageous since the ends of the printing plates must be sufficiently rigidly clamped to each other to avoid slippage of wrongly guided printing plates to be inserted into the clamping device.
It has been also found disadvantageous that not all the thickness of the printing plate takes the parallel position when the plate is inserted between the upper and lower rails which leads to higher local loads exerted on the printing plate by clamping action and therefore rupture or displacement of the printing plate into a clamping gap may occur which makes impossible reuse of the equipment with the truing systems.
Still another disadvantage of the conventional devices is that even not great wear which may occur in bolts with rounded heads sufficiently decreases the clamping force. The operation of the eccentric shaft by a plug spanner is unpractical and requires enough space to install the same on the clamping device.